1. Field of the Invention
The present invention relates to a magnetic resonance imaging (MRI) apparatus and an image processing apparatus which excites nuclear spins of an object magnetically with an RF (radio frequency) signal having the Larmor frequency and reconstructs an image based on an NMR (nuclear magnetic resonance) signal generated due to the excitation, and more particularly, to a magnetic resonance imaging apparatus and an image processing apparatus which can obtain a blood flow image in a shorter period of time without a contrast medium.
2. Description of the Related Art
Magnetic resonance imaging is an imaging method which excites nuclear spins of an object set in a static magnetic field with an RF signal having the Larmor frequency and reconstructs an image based on an NMR signal generated due to the excitation.
MRA (Magnetic Resonance Angiography) is known as a technique for obtaining a blood flow image in the field of magnetic resonance imaging. MRA without using contrast medium is called a non-contrast-enhanced MRA. In non-contrast-enhanced MRA, a fresh blood imaging (FBI) method is designed, which clearly images a blood vessel by acquiring a high-velocity blood flow pumped out from the heart with ECG (electro cardiogram) synchronization (see, for example, Japanese Patent Application (Laid-Open) No. 2000-5144).
A non-contrast-enhanced MRA image using the FBI method can obtain an MRA image with arteriovenous separation by obtaining a difference between image data acquired while varying delay time with ECG synchronization. Additionally, a Flow-Spoiled FBI method can be designed, which suppresses an artery signal during systole by applying a spoiler pulse. The Flow-Spoiled FBI method images a difference between artery signals in diastole and systole of myocardium. An ECG-prep scan can be used to decide upon an optimum delay time for use with the ECG synchronization.
Furthermore, in the FBI method, a Flow-dephasing method can be designed in order to image a low-velocity blood flow, which applies a gradient pulse (Gspoil) in a readout (RO) direction and adds a dephasing pulse or a rephasing pulse to a gradient magnetic field pulse (see, for example, Japanese Patent Application (Laid-Open) No. 2002-200054, Japanese Patent Application (Laid-Open) No. 2003-135430 and U.S. Pat. No. 6,801,800). The Flow-dephasing method can increase a relative signal difference between a high-velocity blood flow and a low-velocity blood flow by operation of a dephasing pulse or a rephasing pulse. Then, the relative signal difference allows one to more clearly separate arteries from veins.
This means it is important to enlarge signal difference between diastole and systole for clear arteriovenous separation. In order to do this, it is necessary to reduce signal intensity from high-velocity blood flow in systole. Therefore, a gradient pulse having an appropriate intensity in an RO direction is set and a blood flow signal from an artery in systole is suppressed by the set gradient pulse. In this state, a blood flow signal in diastole is acquired. Then, subtraction processing and/or maximum intensity projection (MIP) processing is performed on the blood flow signal acquired in diastole and thus only arteries are imaged.
The Flow-prep. Scan performs a preparation scan with changing parameters including intensity of a dephasing pulse in an RO direction in the Flow-dephasing method (see, for example, Japanese Patent Application (Laid-Open) No. 2003-70766). The Flow-prep. Scan makes it possible to obtain an optimum parameter by referencing images acquired with changed parameter values during the preparation scan. A research report about intensity of a dephasing pulse in an RO direction has been made (see, for example, Miyazaki M, et al., Radiology 227:890-896, 2003).
An echoshare technique with use of the Half-Fourier method is designed to obtain a blood flow image in a shorter scanning time (see, for example, Japanese Patent Application (Laid-Open) No. 2001-149341). Scans in diastole and systole obtain pieces of data respectively by this technique. By the scan in systole, only echo data in a low-frequency region important for improving contrast is acquired in a short data acquisition time. Instead, copies of data in a high-frequency region acquired by the scan in diastole are used as data in the high-frequency region that are not acquired in systole. Moreover, if data is still insufficient in a region even with using a copy of data in diastole, then further data is calculated from K-space data for diastole and systole by the Half-Fourier method so as to provide a complete K-space data set for image reconstruction.
A technique to obtain dynamic information of a blood flow simply without contrast medium and measuring ECG-synchronization timing with an ECG-prep scan can be designed (see, for example, Japanese Patent Application (Laid-Open) No. 2004-329614). This technique uses an ECG-prep scan as an imaging scan. Specifically, as in the case of an ECG-prep scan, dynamic information of blood flow can be obtained by performing subtraction processing on two-dimensional data acquired more than once, but by imaging scans made at gradually changed respective delay times from an R wave of an ECG signal.
FIG. 18 is a diagram showing a conventional imaging scan with use of an ECG-prep scan.
In FIG. 18, the abscissa denotes time. As shown in FIG. 18, a trigger signal is set initially at a timing of delay time d1 from an R wave of an ECG signal. Then, a scan for data acquisition is started in synchronization with the trigger signal. Then, a trigger signal is set at a delay time d2 from an R wave of the ECG signal after completion of the scan for data acquisition. Then, a scan for data acquisition is started in synchronization with the trigger signal. In the same way, trigger signals are set at delay times d3, d4, . . . from R waves of the ECG signal respectively, and a scan for data acquisition is started at a different timing in synchronization with each trigger signal. At this time, delay times d1, d2, . . . are set to be changed gradually in systole of a myocardium showing a great change in velocity of blood flow.
A pulse sequence that consists of a two-dimensional partial FS (flow spoiling) pulse and/or a two-dimensional partial FC (flow compensation) pulse is used as a data acquisition scan.
The imaging scan as shown in FIG. 18 can reconstruct multiple pieces (i.e., sets) of image data each corresponding to a mutually different delay time from an R wave of the ECG signal.
FIG. 19 is a diagram showing delay times from an R wave of an ECG signal for respective acquisition timings of pieces of image data acquired by the imaging scan shown in FIG. 18.
In FIG. 19, the abscissa denotes time and each arrow denotes a timing of an R wave of an ECG signal. As shown in FIG. 19, multiple pieces of image data each corresponding to a mutually different delay time from an R wave of the ECG signal are generated by an imaging scan. This means multiple pieces of image data each corresponding to a mutually different cardiac time phase are generated. A subtraction image obtained by performing subtraction processing to the pieces of image data generated in this way becomes a blood flow image presenting dynamic information of a blood flow. This blood flow image is called the Time-resolved MRDSA (magnetic resonance digital subtraction angiography) image since it is a subtraction image of a blood flow resolved by time.
A technique can also be designed to obtain dynamic information of blood flow mentioned above by PI (parallel imaging) which is one of the available high speed imaging methods. PI is a technique for using a PAC (phased array coil) having surface coils as an RF coil for data reception (see, for example, Japanese Patent Application (Laid-Open) No. 2004-329613). By using PI for obtaining dynamic information of blood flow, Ran scanning time can be reduced.
In a conventional technique for obtaining a Time-resolved MRDSA image, data acquisition time per 1 shot to acquire Time-resolved MRDSA data used for generating a Time-resolved MRDSA image is considerably long compared to typical intervals between R waves. Therefore, a TR (repetition time) is set to be about 3RR corresponding to 3 times of a distance RR between R waves.
Consequently, an imaging time of about 60 to 90 seconds is necessary to generate different Time-resolved MRDSA images corresponding to 20 to 30 time phases. Therefore, shortening of imaging time is needed.
In addition to shortening of imaging time, when scanned data is processed simply and appropriately in accordance with the diagnostic purpose and a user can refer to a diagnostic image more easily, working time and diagnostic time of the user can be reduced even if an imaging time might increase.